Crystal Ball Fisheye for Canon PowerShot ELPH180

It's fairly obvious that a crystal ball produces a fisheye view when you look into it... so why not have a camera look into it? That's what this thing does: mount a little crystal ball at an appropriate distance in front of a Canon PowerShot ELPH180 (or similar camera).

Clear crystal balls are very common on eBay; a 30mm diameter one sells for about $2. Unfortunately, the one I got has a number of nicks and inclusions, and it isn't perfectly round. There is also the little problem that DoF o